Plants with leaves all year round are evergreens, and those that shed their leaves are deciduous. Deciduous trees and shrubs generally lose their leaves in autumn. Before this happens, the leaves change colour. The leaves will grow back in spring.

The epidermis is the outer layer of cells covering the leaf. It forms the boundary separating the plant's inner cells from the external world.

The epidermis is covered with pores called stomata. They are part of a complex with a pore surrounded on each side by chloroplast-containing guard cells, and two to four subsidiary cells that lack chloroplasts. Opening and closing of the stoma complex regulates the exchange of gases and water vapor between the outside air and the interior of the leaf and plays an important role in allowing photosynthesis without letting the leaf dry out.

Most of the interior of the leaf between the upper and lower layers of epidermis is a tissue called the mesophyll (Greek for "middle leaf"). This assimilation tissue is the main place photosynthesis takes place in the plant. The products of photosynthesis are sugars, which can be turned into other products in plant cells.

In ferns and most flowering plants, the mesophyll is divided into two layers:

An upper palisade layer of tightly packed, vertical cells, one to two cells thick. Its cells contain many chloroplasts. The chloroplasts move by a process called "cyclosis". The slight separation of the cells provides maximum absorption of carbon dioxide. Sun leaves have a multi-layered palisade layer, while shade leaves closer to the soil are single-layered.

Beneath the palisade layer is the spongy layer. The cells of the spongy layer are more rounded and not so tightly packed. There are large air spaces between the cells. These cells contain fewer chloroplasts than those of the palisade layer. The pores or stomata of the epidermis open into chambers, which are connected to the air spaces between the spongy layer cells.

Many leaves are covered in trichomes (small hairs) which have a wide range of structures and functions. Some trichomes are prickles, some are scaled, some secrete substances such as oil. Carnivorous plants secrete digestive enzymes from trichomes.

What leaves look like on the plant varies greatly. Closely related plants have the same kind of leaves because they have all descended from a common ancestor. The terms for describing leaf shape and pattern is shown, in illustrated form, at Wikibooks.

Different terms are usually used to describe leaf placement (phyllotaxis):

The leaves on this plant are arranged in pairs opposite one another, with successive pairs at right angles to each other ("decussate") along the red stem. Note the developing buds in the axils of these leaves.

Alternate — succeessive leaves in alternate direction along the stem.

Opposite — Two structures, one on each opposite side of the stem, typically leaves, branches, or flower parts.

Whorled — three or more leaves attach at each point or node on the stem.

Leaves form a helix pattern centered around the stem, with (depending upon the species) the same angle of divergence. There is a regularity in these angles and they follow the numbers in a Fibonacci sequence. This tends to give the best chance for the leaves to catch light.

Two basic forms of leaves can be described considering the way the blade (lamina) is divided.

A simple leaf has an undivided blade. However, the leaf shape may be formed of lobes, but the gaps between lobes do not reach to the main vein.

A compound leaf has a fully subdivided blade, each leaflet of the blade separated along a main or secondary vein. Because each leaflet can appear to be a simple leaf, it is important to recognize where the petiole occurs to identify a compound leaf. Compound leaves are a characteristic of some families of higher plants, such as the Fabaceae. The middle vein of a compound leaf or a frond, when it is present, is called a rachis.

Some leaves have a petiole (leaf stem). Sessile leaves do not: the blade attaches directly to the stem. Sometimes the leaf blad surrounds the stem, giving the impression that the shoot grows through the leaf.

In some Acacia species, such as the Koa Tree (Acacia koa), the petioles are expanded or broadened and function like leaf blades; these are called phyllodes. There may or may not be normal pinnate leaves at the tip of the phyllode.

Poinsettiabracts are leaves which have evolved red pigmentation in order to attract insects and birds to the central flowers, an adaptive function normally served by petals (which are themselves leaves highly modified by evolution).

In the course of evolution, many species have leaves which are adapted to other functions.